The Biology of Hydrothermal Vent Ecosystems

Alviniconcha sp. in high concentration at hydrothermal vent site | Source

Hydrothermal
vents support some of the most productive and densely populated marine
biological communities on the planet, but species diversity is low when
compared to non-chemosynthetic-based benthic communities (Ramirez-Llodra et el
2007, Glowka 2003). Most hydrothermal ecosystems are comprised of a small
number of large organisms that rely mostly on symbioses with chemoautotrophic
bacteria for energy, and a larger group of small heterotrophs that are
primarily carnivorous, detritivorous or suspension-feeding (Bachraty et el
2009). The most specious taxon of organisms capable of living in the
hydrothermal environment is Arthropoda , and these animals make up a high
percentage of the total vent biomass (Ramirez-Llodra et el 2007). Invertebrate
species thriving at hydrothermal sites include limpets, barnacles,
vestimentiferan tubeworms, bivalve mollusks, provannid gastropods and bresiliid
shrimp (Van Dover et el 2002).

From
an evolutionary standpoint, dominant vent taxa (vestimentiferan tubeworms,
bathymodiolid mussles, vesicomyid clams and bressiilid shrimp) are relatively
young. Molecular clock data shows that most vent fauna evolved to its current
state less than 100 million years ago during the late Cretaceous and Cenozoic
(Bachraty et el 2009, Van Dover et el 2002). Current research suggests that
many neighboring vent sites have organisms that are genetically connected,
which indicates interbreeding between metapopulations. This provides a degree
of resilience (at the species level) to events that affect individual vent
sites (Devey et el 2007). However, patchy distributions along spreading centers
can cause the isolation of groups of species and allopatric speciation has been
an important factor in differentiating hydrothermal vent provinces (Bachraty et
el 2009, Van Dover et el 2002).

Most
vent invertebrates colonize suitable habitats as larvae dispersed by deep sea
currents, and are characterized by rapid rates of both population growth and
colonization. (Van Dover et el 2002). Hydrothermal vent fauna have evolved to
enhance colonization in a variety of ways. Some mussel species remove sulfide
from the vent effluent and act as diffusing agents, dispersing vent fluids
laterally for several meters, increasing the area of available habitat (Van
Dover & Lutz 2004). The average distance of dispersal for the larva of most
vent species is 35 to 55 km over a time period of two weeks. The larva of some
polychaeats go into developmental arrest in cold water, extending both
longevity and dispersal distance (Van Dover et el 2002).

The
extreme conditions characteristic of hydrothermal vents has led to high
instances of endemism. Approximately 85 percent of known vent species are
endemic (Ramirez-Llodra et el 2007). High taxonomic levels of endemicity
suggests the evolutionary origin of certain vent species could date back as
late as the Paleozoic (Van Dover et el 2002). Both endemic and non-native
species have made morphological, physiological and ecological adaptations to
adjust to the hydrothermal environment. Many animals have complex combinations
of metal detoxification systems that allow them to survive (Gonzales-Rey et el
2008). Others, such as gastropods Alviniconcha sp . and Infremeria nautilei,
have an anatomical stomach volume that is 1/10 that of other provannid
gastropods, as well as hypertrophied gills that enhance oxygen uptake. Larger
gill area also provides more room for endosymbionts which usually live in the
epithelial gill tissue of mollusk species (Henry et el 2008). Certain
tubeworms, such as R. pachyptila , have physiologies that allow their bodies to
span the anoxic and aerobic zones of vent areas. These areas can also differ
significantly in temperature. Some worms have adapted to live with one end of
their body in water 50 C cooler than the other (Fisher et el 2007, Phillips
2006).

Thermarces cerberus

Species Case Study: Thermarces cerberus

There are significantly more invertebrate species
inhabiting hydrothermal ecosystems than vertebrate, partially due to the slower
rate of genetic evolution and adaptation associated with higher taxonomic
groups. The zoarcid fish Thermarces cerberus is one of the few vertebrate species
endemic to vent environments and can be found in areas near the East Pacific
Rise. T. cerberus feeds on mollusks and crustaceans that are thought to synthesize
essential fatty acids necessary to all vertebrate diets in situ. These fatty
acids are usually produced by photosynthetic microplankton in the photic zone
of the ocean and do not typically reach the benthos of the deep sea, suggesting
that T. cerberus is obtaining them from a source in the hydrothermal
environment. Sulfur-oxidizing bacterial symbionts of vent species are not known
to produce large polyunsaturated fatty acid molecules, which indicates the
invertebrates may have evolved biochemical mechanisms once thought exclusive to
photosynthetic organisms (Pond et el 2008).

Source

Species Case Study: Riftia pachyptila

Spanning as long as six feet, R. pachyptila is one of the
most prominent and widely recognized members of the hydrothermal community. The
tubeworm depends on its intracellular chemoautotrophic symbionts for 100 percent of its nutritional
needs, and has adapted specific traits to enhance the efficiency of this
relationship. R. pachyptila does not have a mouth, gut or anus. Instead it has
developed a large internal organ, the trophosome, where it houses its
symbionts. These chemoautotrophs generate organic carbon molecules for the worm
to feed upon. Through its plume, R. pachyptila absorbs carbon dioxide, hydrogen
sulfide and oxygen from the surrounding environment. The tubeworm has developed
a unique hemoglobin that binds reversibly to both oxygen and hydrogen sulfide,
the substrates needed by the symbionts to generate energy molecules the worm is
capable of utilizing. Remarkably, this unique species has also developed a
resistance to toxic effects normally observed when hydrogen sulfide is
transported through blood and tissue (Fisher & Girguis 2007, Fisher et el
2007, Ramirez-Llodra 2007, Phillips 2006).